If indeed there is no difference at all, that both phones get the same quality of driver, then it is quite shocking to think people buying the 595's aren't getting better quality drivers, for their extra money paid. £40-50 extra for what exactly?
Would be interesting to know if the 555's do get not up to par drivers, that don't qualify for the 595's, or if it's solely down to the housing construction. Suppose we will never know.
ATi did, or probably still do something similar. Maybe it's standard for many companies. An older line of their cards, they used the same chip for the top model and for a lesser model. The chips that didn't make the grade go into the lesser model.